The present invention relates, in general, to improved variable reactance transformers, and more particularly to a variable reactance transformer construction which permits use of the transformers in a three-phase delta configuration without stress failure.
Controllable transformers which utilize the saturating effects of direct current to derive a controllable output voltage and current have been known in the prior art, and one such transformer is disclosed in U.S. Pat. No. 3,343,074, issued Sept. 19, 1967 to Elwood M. Brock. In that patent a variable reactance transformer is shown as having three annular, concentrically arranged cores carrying control, primary and secondary windings formed as toroids on the cores. Each of the two outside or auxiliary cores carries a DC winding, the central or main core carries the secondary winding, while the primary winding encircles all three cores. The DC windings are connected in series with a variable source of direct current control power by which the saturation level of the control cores may be regulated. Variation of the state of saturation of the control cores presents a selectable reactance to the primary current, and thereby effects control of the output of the secondary winding. The two control windings are connected in series opposition to provide control on both halves of the alternating current in the primary winding.
The structure illustrated in U.S. Pat. No. 3,343,074 provides a variable transformer having an improved response time, an improved power factor and by substantially eliminating flux leakage, provides greater efficiency than was possible with prior art devices. Further, the reactor provides substantial savings in both material and labor to provide a considerable reduction in the costs of manufacture. The device produces efficient coupling and a very low reactance in its fully saturated condition, and provides an extremely high reactance in its unsaturated condition.
Although the transformer described in the foregoing patent has been found to be extremely satisfactory in most applications, and has met with considerable commercial success, it was found that when the unit was connected in a three-phase system of relatively high power, such as might be required in the operation of an electric furnace or similar load conditions, occasional failures of the transformers were experienced. Although these failures appeared, at first, to be random, it was found that in each case the failure involved a breakdown of the insulation between the control and primary windings in the transformers, and involved two of the three transformers in a three-phase system. A stress analysis of these units indicated that the breakdowns were being caused by very high voltages which exceeded by substantial margins the capabilities of the insulation being used. Such failures principally occurred when the DC control windings of the three transformers were connected externally in parallel to a single control source, and when the primaries of the transformers were connected together in a delta arrangement.
Even after a great deal of study and testing of the transformers, it still was not possible to pinpoint the cause of such failures. However, a careful analysis of the voltage equivalent circuits of the transformer suggested that the induction of alternating current from the primary into the control windings was causing the problem, and that the difficulty was compounded when the physical relationship of the control and primary windings was such that the beginning, or start, of one winding was adjacent the ending, or finish, of the other, with the physical relationship being such that the AC voltage induced across the DC winding was additive to the AC voltage across the primary winding. The stress produced by this additive voltage was found, upon analysis, to be greater than the insulation could handle, resulting in failure of the transformers. Such failures were calamitous, not only because the failures resulted in the total loss of two of the three transformers in each three-phase system in which they were installed, but because such failures resulted in shutdown of the system with resultant losses of material, losses of labor and time in replacing the transformers, and loss of business to the users.
One immediately apparent solution to the problem was to increase the insulation within the transformer to prevent such stress breakdown. However, an analysis of the voltage levels causing the stress failures indicated that in a transformer nominally designed to handle 480 volts AC it would have been necessary to provide insulation that would withstand over 9,800 volts in order to meet the standard NEMA requirements for insulation. These requirements are that the insulation to able to withstand twice the nominal voltage, plus 1,000. In this case, then, the nominal strenth requirement of the insulation would have been that it withstand 2 times 480 volts plus 1,000 volts, or 1,960 volts. Thus, it was apparent that a five fold increase in the insulation within the transformer would have been required to prevent the stress failures, and such an increase would have substantially eliminated the variable reactance transformer from consideration as a three-phase source in such systems, since that amount of insulation would have made it economically unfeasible as well as bulky, heavy, and difficult to manufacture and install.